instructiont::transform applies to function in function call

We previously only transformed the left-hand side and arguments, but
skipped over the function itself. This omission became apparent when
trying to run the upcoming SIMD1 test (added in a later commit, further
bug fixes are necessary).

Author: tautschnig

src/goto-programs/goto_program.cpp

@@ -974,6 +974,13 @@ void goto_programt::instructiont::transform(
       change = true;
     }
 
+    auto new_function = f(new_call.function());
+    if(new_function.has_value())
+    {
+      new_call.function() = *new_function;
+      change = true;
+    }
+
     for(auto &a : new_call.arguments())
     {
       auto new_a = f(a);